The present invention relates to an improved mix of carbon aggregate and binder. More particularly, it relates to a seam mix applicable at room temperature for forming fluid-tight, electrically conductive seams between carbon blocks in the bottom of a Hall-Heroult cell for producing aluminum metal by the electrolysis of Al.sub.2 O.sub.3.
Background material on the technique of using seam mixes in the construction of Hall-Heroult cells is given in U.S. Pat. No. 2,378,142, issued June 12, 1945, to Hurter for a "Method for Making Furnaces for the Electrolytic Production of Aluminum" and in U.S. Pat. No. 3,576,700, issued Apr. 27, 1971, to M. B. Dell for "High Strength Joined Porous Block Structure and Method of Making Same". The seams are referred to in the Hurter patent as "interspaces".
According to the present method of seam mix usage in the construction of Hall-Heroult cells, prebaked cathode blocks of carbon are arranged in a cell bottom, separated by seams one to three inches wide. To obtain optimum properties, e.g. high density in the seam, conventional practice has been to preheat the carbon blocks to 60.degree. to 130.degree. C. with hot, for example, 130.degree. C., seam mix then being incrementally tamped into the seams and between the cell side lining and the blocks to provide a smooth cell bottom.
This practice is disadvantageous for a number of reasons. Fumes are evolved from the heated mix. Considerable manpower and coordination are needed to achieve the placement of the hot seam mix between the heated blocks before the mix has had a chance to cool. The elevated temperatures mean discomfort for the people performing the work. In addition, significant capital and operating costs are associated with the preheating of both the blocks and the seam mix.
A composition used for making molded carbon bodies but which has no utility as a room temperature seam mix is described in Greaves et al U.S. Pat. No. 2,637,072. The Greaves composition includes coal not subjected to calcining and containing 10-15% volatile matter, thereby resulting in excessive shrinkage upon heating to 950.degree. C. (see Example III below). A preferred formula for the Greaves carbon bodies contains coal tar pitch having a softening point of 105.degree. C. and is molded at a temperature of 100.degree. C., rather than at room temperature.